Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 721: Insulin resistance down-regulates the translocation of which glucose transporter to the cell membrane?

A. GLUT-1
B. GLUT-2
C. GLUT-3
D. GLUT-4 (Correct Answer)

Explanation: ***GLUT-4*** - **Insulin resistance** primarily affects cells that express **GLUT-4**, such as muscle and adipose tissue, by impairing its translocation from intracellular vesicles to the cell membrane. - This reduced translocation leads to decreased glucose uptake in response to insulin, a hallmark of **type 2 diabetes**. *GLUT-1* - **GLUT-1** is responsible for basal glucose uptake in nearly all cells, including **erythrocytes** and endothelial cells of the blood-brain barrier. - Its activity is largely **insulin-independent** and not significantly affected by insulin resistance in the same way as GLUT-4. *GLUT-2* - **GLUT-2** is found primarily in **pancreatic β-cells**, hepatocytes, renal tubular cells, and enterocytes. - It has a low affinity but high capacity for glucose transport, playing a key role in **glucose sensing** and facilitating glucose flux in and out of these cells, independent of insulin translocation. *GLUT-3* - **GLUT-3** is predominantly expressed in **neurons** and the placenta, where it facilitates high-affinity glucose uptake. - It is crucial for maintaining the brain's glucose supply and its activity is also **insulin-independent**.

Question 722: Which enzyme converts glucose to sorbitol?

A. Sorbitol dehydrogenase
B. Aldose reductase (Correct Answer)
C. Aldolase B
D. Glucose-6-phosphate dehydrogenase

Explanation: ***Aldose reductase*** - This enzyme is crucial in the **polyol pathway**, reducing **glucose to sorbitol** by using **NADPH** as a cofactor. - In conditions of high glucose (e.g., uncontrolled diabetes), increased activity of **aldose reductase** leads to sorbitol accumulation, contributing to **osmotic damage** in certain tissues like the lens, nerves, and kidneys. *Sorbitol dehydrogenase* - This enzyme is responsible for the subsequent step in the polyol pathway, **oxidizing sorbitol to fructose** using NAD+ as a cofactor. - While related to sorbitol metabolism, it does not convert glucose to sorbitol; instead, it metabolizes sorbitol further. *Aldolase B* - This enzyme is involved in **fructose metabolism**, specifically cleaving **fructose-1-phosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde**. - It plays no direct role in the conversion of glucose to sorbitol. *Glucose-6-phosphate dehydrogenase* - This is the rate-limiting enzyme of the **pentose phosphate pathway**, catalyzing the oxidation of glucose-6-phosphate to 6-phosphoglucono-δ-lactone. - While it also uses NADPH (producing it rather than consuming it), it is not involved in the polyol pathway or sorbitol synthesis.

Question 723: Which of the following enzymes do not catalyze an irreversible step in glycolysis?

A. Pyruvate kinase
B. Phosphofructokinase
C. Hexokinase
D. Phosphoglycerate kinase (Correct Answer)

Explanation: ***Phosphoglycerate kinase*** - This enzyme catalyzes the conversion of **1,3-bisphosphoglycerate** to **3-phosphoglycerate**, generating ATP. - This reaction is considered reversible because the free energy change is close to zero under physiological conditions, allowing for both forward (glycolysis) and reverse (gluconeogenesis) flux. *Hexokinase* - This enzyme catalyzes the **irreversible phosphorylation** of glucose to glucose-6-phosphate, trapping glucose within the cell. - It is one of the key regulatory enzymes in glycolysis, and its irreversibility ensures that glucose uptake and phosphorylation proceed in one direction. *Pyruvate kinase* - This enzyme catalyzes the final, **irreversible step** of glycolysis, converting phosphoenolpyruvate to pyruvate and generating ATP. - This reaction is a major control point for glycolysis due to its large negative free energy change. *Phosphofructokinase* - This enzyme catalyzes the **irreversible phosphorylation** of fructose-6-phosphate to fructose-1,6-bisphosphate. - It is considered the **rate-limiting step** and a primary control point in glycolysis, making it highly regulated and unidirectional.

Question 724: In which of the following pathways is UDP glucose not utilized?

A. Uronic acid pathway
B. Glycogen synthesis
C. Galactose metabolism
D. HMP shunt (Correct Answer)

Explanation: ***HMP shunt*** - The **Hexose Monophosphate Shunt (HMP shunt)**, also known as the **pentose phosphate pathway**, primarily uses **glucose-6-phosphate** as its substrate. - Its main products are **NADPH** and **ribose-5-phosphate**, and it does not involve **UDP-glucose**. *Uronic acid pathway* - The **uronic acid pathway** converts **glucose** to **glucuronic acid**, **L-xylulose**, and **ascorbic acid (in some animals)**, utilizing **UDP-glucose** as an intermediate. - Specifically, **UDP-glucose dehydrogenase** oxidizes UDP-glucose to **UDP-glucuronate**. *Glycogen synthesis* - In **glycogen synthesis (glycogenesis)**, **UDP-glucose** is the direct precursor for adding glucose units to the growing **glycogen chain**. - The enzyme **glycogen synthase** catalyzes the transfer of glucose from UDP-glucose to the non-reducing end of glycogen. *Galactose metabolism* - In **galactose metabolism**, **UDP-glucose** plays a crucial role in the conversion of **galactose-1-phosphate** to **glucose-1-phosphate**. - This occurs via the enzyme **galactose-1-phosphate uridyltransferase**, which exchanges UDP from UDP-glucose with the phosphate from galactose-1-phosphate, forming **UDP-galactose** and **glucose-1-phosphate**.

Question 725: What is the cause of lactose intolerance?

A. Deficiency of Galactokinase
B. Deficiency of Uridyl transferase
C. Deficiency of Lactase (Correct Answer)
D. Deficiency of Enteropeptidase

Explanation: ***Deficiency of Lactase*** - Lactose intolerance results from the insufficient production of the enzyme **lactase**, which is responsible for breaking down **lactose** (a disaccharide found in milk and dairy products) into glucose and galactose. - When lactase is deficient, undigested lactose passes into the colon, where it is fermented by bacteria, leading to symptoms like **bloating**, **gas**, **diarrhea**, and **abdominal pain**. *Deficiency of Galactokinase* - A deficiency in **galactokinase** causes **Type II galactosemia**, a disorder involving the inability to metabolize galactose. - This condition primarily leads to **cataracts** and does not directly cause the digestive symptoms associated with lactose intolerance. *Deficiency of Uridyl transferase* - A deficiency in **uridyl transferase** causes **classic galactosemia (Type I)**, the most severe form of galactosemia. - This condition results in a buildup of toxic galactose metabolites, leading to **liver damage**, **renal failure**, and **developmental delay**, not lactose intolerance. *Deficiency of Enteropeptidase* - **Enteropeptidase** (also known as enterokinase) is an enzyme in the small intestine that activates trypsinogen to trypsin, which then activates other pancreatic proteases. - A deficiency leads to **protein malabsorption** and failure to thrive, not the fermentation of lactose by gut bacteria.

Question 726: Which of the following is an aldose sugar?

A. Ribulose
B. Fructose
C. Glyceraldehyde (Correct Answer)
D. None of the options

Explanation: ***Glyceraldehyde*** - **Glyceraldehyde** is the simplest **aldose**, a monosaccharide with an **aldehyde group** (CHO) at one end of its carbon chain. - Its chemical structure is a three-carbon chain with the aldehyde group on the first carbon, making it an **aldo sugar**. *Ribulose* - **Ribulose** is a **ketose**, specifically a **ketopentose**, meaning it is a five-carbon sugar with a **ketone group** (C=O) in its structure. - The ketone group in ribulose is typically located on the second carbon, distinguishing it from aldoses. *Fructose* - **Fructose** is another example of a **ketose**, specifically a **ketohexose**, as it is a six-carbon sugar containing a **ketone group**. - Its ketone group is usually found on the second carbon atom, which differentiates ketoses from aldoses structurally. *None of the options* - This option is incorrect because **glyceraldehyde** is indeed an aldose sugar, fitting the definition of a monosaccharide with an aldehyde functional group. - As **glyceraldehyde** is correctly identified as an aldose, this choice would contradict the chemical classification of sugars.

Question 727: Which of the following dietary fibers is most characteristically insoluble in water?

A. Pectin
B. Lignin (Correct Answer)
C. Hemicellulose
D. Cellulose

Explanation: ***Lignin*** - **Lignin** is a complex polymer found in plant cell walls, known for its **extreme insolubility** in water. - It provides structural rigidity to plants and is a non-carbohydrate component of **dietary fiber**. *Pectin* - **Pectin** is a type of soluble dietary fiber that forms a **gel-like substance** when mixed with water. - It is often used as a gelling agent in foods and is found in fruits like apples and citrus. *Hemicellulose* - **Hemicellulose** is a diverse group of polysaccharides; some forms are **soluble**, while others are **insoluble**, but it's generally more soluble than lignin. - Its solubility depends on its specific structure and sugar composition. *Cellulose* - **Cellulose** is an insoluble fiber, but it can absorb water and swell, contributing to **bulk in stool**. - While largely insoluble, **lignin** is considered the most characteristically insoluble fiber due to its highly cross-linked and rigid structure, which resists hydration even more effectively than cellulose.

Question 728: When the insulin to glucagon ratio decreases, which enzyme is primarily active?

A. Glucose-6-phosphatase
B. Pyruvate carboxylase
C. Fructose 1,6-bisphosphatase
D. Glycogen phosphorylase (Correct Answer)

Explanation: ***Glycogen phosphorylase*** - A decrease in the **insulin-to-glucagon ratio** indicates a **low blood glucose** state, signaling the need for glucose production. - **Glycogen phosphorylase** is the key enzyme in **glycogenolysis**, which breaks down stored glycogen into glucose-1-phosphate, thereby elevating blood glucose. - This is the **primary and fastest response** to decreased insulin/glucagon ratio. *Fructose-1,6-bisphosphatase* - This enzyme is crucial for **gluconeogenesis**, specifically catalyzing the dephosphorylation of **fructose-1,6-bisphosphate** to **fructose-6-phosphate**. - While active during low insulin/high glucagon states, it is involved in synthesizing glucose, not directly breaking down stored glycogen as quickly as glycogen phosphorylase. *Pyruvate carboxylase* - This enzyme is the first committed step in **gluconeogenesis**, converting **pyruvate to oxaloacetate** in the mitochondria. - Although active in response to a low insulin-to-glucagon ratio, its role is in synthesizing glucose from non-carbohydrate precursors, which is a slower process than immediate glycogen breakdown. *Glucose-6-phosphatase* - This enzyme is found primarily in the **liver and kidneys** and is responsible for dephosphorylating **glucose-6-phosphate** to free glucose, allowing it to exit the cell into the bloodstream. - While essential for the release of glucose from both gluconeogenesis and glycogenolysis, it acts at a later stage to make glucose available rather than initiating the breakdown of glycogen itself.

Question 729: An adolescent male patient presents to you with exercise intolerance. He gives a history of developing cramps on exertion. Which of the following enzyme deficiencies could be the cause?

A. Myophosphorylase (Correct Answer)
B. Hexokinase
C. Glucose-6-phosphatase
D. Hepatic glycogen phosphorylase

Explanation: ***Myophosphorylase*** - A deficiency in **myophosphorylase** (McArdle's disease, Glycogen Storage Disease Type V) impairs muscle glycogen breakdown, leading to **exercise intolerance** and **muscle cramps** due to insufficient ATP production during exertion. - Patients often experience a "second wind" phenomenon where symptoms improve after resting, as free fatty acids become an alternative fuel source. *Hexokinase* - A deficiency in **hexokinase** would affect the first step of glycolysis, impacting glucose phosphorylation in all tissues, not specifically causing exercise-induced muscle cramps. - This deficiency is rare and typically presents with **hemolytic anemia** due to impaired erythrocyte metabolism. *Glucose-6-phosphatase* - A deficiency in **glucose-6-phosphatase** (Von Gierke's disease, Glycogen Storage Disease Type Ia) primarily affects the liver and kidneys, leading to **fasting hypoglycemia**, lactic acidosis, and hepatomegaly, not exercise intolerance. - Muscle glycogen metabolism is unaffected in this condition. *Hepatic glycogen phosphorylase* - A deficiency in **hepatic glycogen phosphorylase** (Hers' disease, Glycogen Storage Disease Type VI) mainly causes **hepatomegaly** and **mild hypoglycemia** because the liver cannot effectively mobilize its glycogen stores. - **Muscle glycogen metabolism** remains normal, so exercise intolerance and cramps are not characteristic symptoms.

Question 730: Classical galactosemia (Type I) is due to deficiency of which enzyme?

A. Galactose-1-phosphate uridyltransferase (Correct Answer)
B. Fructose-1,6-bisphosphatase
C. Adenine phosphoribosyltransferase (APRT)
D. Hexokinase

Explanation: ***Galactose-1-phosphate uridyltransferase*** - **Galactosemia Type I** (**classical galactosemia**) is caused by a deficiency in **galactose-1-phosphate uridyltransferase (GALT)**. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway of galactose metabolism. *Adenine phosphoribosyltransferase (APRT)* - Deficiency in **adenine phosphoribosyltransferase (APRT)** leads to **APRT deficiency**, characterized by **kidney stones** composed of 2,8-dihydroxyadenine. - This enzyme is involved in **purine salvage pathways**, not carbohydrate metabolism. *Fructose-1,6-bisphosphatase* - A deficiency in **fructose-1,6-bisphosphatase** causes **fructose-1,6-bisphosphatase deficiency**, a disorder of **gluconeogenesis**. - It results in **hypoglycemia** and **lactic acidosis**, especially during fasting. *Hexokinase* - **Hexokinase** phosphorylates glucose to **glucose-6-phosphate**, the first step in glycolysis. - Deficiency is rare but can lead to **nonspherocytic hemolytic anemia**.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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